Checkpoints - Sentences should be a maximum of 25 words
Studies also show that sentences of 11 words are considered easy to read, while those of 21 words are fairly difficult. At 25 words, sentences become difficult, and 29 words or longer, very difficult.
People with moderate learning disabilities can understand sentences of 5 to 8 words without difficulty. By using common words we can help all users understand sentences of around 25 words. Long sentences aren't just difficult for people who struggle with reading or have a cognitive disability like dyslexia or attention deficit hyperactivity disorder. They're also a problem for highly literate people with extensive vocabularies.
Accessibility (WCAG 2.0 Level A, 3.1.5), Usability
The checkpoint examines the entire HTML source code of a page and reports if any sentences are longer than 25 words.